callback - the AsyncCallback will be invoked when the processing of the exchange is completed.
If the exchange is completed synchronously, then the callback is also invoked synchronously.
The callback should therefore be careful of starting recursive loop.

doStop

Implementations override this method to support customized start/stop.

Important: Camel will invoke this ServiceSupport.doStop() method when
the service is being stopped. This method will also be invoked
if the service is still in uninitialized state (eg has not
been started). The method is always called to allow the service
to do custom logic when the service is being stopped, such as when
CamelContext is shutting down.